Convert `brew command` test to spec.

Library/Homebrew/test/cmd/command_spec.rb

+describe "brew command", :integration_test do
+  it "returns the file for a given command" do
+    expect { brew "command", "info" }
+      .to output(%r{#{Regexp.escape(HOMEBREW_LIBRARY_PATH)}/cmd/info.rb}).to_stdout
+      .and be_a_success
+  end
+
+  it "fails when the given command is unknown" do
+    expect { brew "command", "does-not-exist" }
+      .to output(/Unknown command/).to_stderr
+      .and be_a_failure
+  end
+end

Library/Homebrew/test/command_test.rb

-require "testing_env"
-
-class IntegrationCommandTestCommand < IntegrationCommandTestCase
-  def test_command
-    assert_equal "#{HOMEBREW_LIBRARY_PATH}/cmd/info.rb",
-                 cmd("command", "info")
-
-    assert_match "Unknown command",
-                 cmd_fail("command", "I-don't-exist")
-  end
-end